Liquid crystal display apparatuses have been widely used in clocks, electronic calculators, various home appliances, measuring equipment, panels for automobiles, word processors, electronic personal organizers, printers, computers, TVs, and the like. Common examples of liquid crystal display methods include a TN (twisted nematic) type, a STN (super-twisted nematic) type, a DS (dynamic light scattering) type, a GH (guest-host) type, an IPS (in-plane switching) type, an OCB (optically compensated birefringence) type, an ECB (electrically controlled birefringence) type, a VA (vertical alignment) type, a CSH (color super-homeotropic) type, and a FLC (ferroelectric liquid crystal). There has been a shift in the driving method used from a conventional static driving to a multiplex driving, which has been commonly employed. Simple-matrix liquid crystal displays and, recently, active-matrix (AM) liquid crystal displays, which are driven using a TFT (thin-film transistor), a TFD (thin-film diode), or the like, have been widely employed.
As illustrated in FIG. 1, a common liquid crystal display apparatus includes two substrates (1) each including an alignment film (4); a transparent electrode layer (3a) serving as a common electrode and a color filter layer (2), which are interposed between one of the alignment film and the corresponding substrate; and a pixel electrode layer (3b) interposed between the other alignment film and the corresponding substrate. The two substrates are arranged so that the alignment films face each other, and a liquid crystal layer (5) is held therebetween.
The color filter layer includes a color filter constituted by a black matrix, a red-colored layer (R), a green-colored layer (G), a blue-colored layer (B), and, as needed, a yellow-colored layer (Y).
The amount of impurities contained in a liquid crystal material constituting the liquid crystal layer is strictly controlled because any impurities remaining in the liquid crystal material would greatly affect the electrical characteristics of the display apparatus. It is known that the material constituting the alignment film also affects the electrical characteristics of the liquid crystal layer because any impurities remaining in the alignment film, which is in direct contact with the liquid crystal layer, would migrate into the liquid crystal layer. Thus, the characteristics of the liquid crystal display apparatus due to impurities contained in a material of the alignment film is currently being studied.
As well as a material of the alignment film, a material of the color filter layer, such as an organic pigment, is also considered to affect the liquid crystal layer due to impurities contained in the material of the color filter layer. The direct effect of a material of the color filter layer on the liquid crystal layer has been considered to be very small compared with the effect of a material of the alignment film since the alignment film and the transparent electrode are interposed between the color filter layer and the liquid crystal layer. However, the thickness of the alignment film is generally 0.1 μm or less, and the thickness of the transparent electrode serving as a common electrode disposed on the color-filter-layer side is generally 0.5 μm or less, even in the case where the thickness of the transparent electrode is increased in order to increase electrical conductivity. Therefore, it cannot be said that the color filter layer and the liquid crystal layer are in an environment where they are completely isolated from each other. Consequently, the impurities contained in the color filter layer, which migrate via an alignment film and a transparent electrode, may reduce the voltage holding ratio (VHR) of the liquid crystal layer and may increase the ion density (ID) in the liquid crystal layer, which results in faulty display such as white missing pixels, alignment inconsistencies, and burn-in.
In order to address the faulty display caused by impurities contained in pigments constituting the color filter, a method of controlling elution of impurities into a liquid crystal by using a pigment such that the proportion of a substance extracted from the pigment with ethyl formate is set to be equal to or less than a specific value (PTL 1) and a method of controlling elution of impurities into a liquid crystal by specifying a pigment contained in a blue colored layer (PTL 2) have been studied. However, there is not a great difference between these methods and a method of simply reducing the amount of impurities contained in a pigment, and these methods provide unsatisfactory improvements in addressing the faulty display in the present situation in which progress has been made in purification techniques for pigments.
On the other hand, focusing on the relationship between organic impurities contained in the color filter and the liquid crystal composition, a method in which the degree of difficulty in dissolving organic impurities in the liquid crystal layer is represented as a hydrophobicity parameter of liquid crystal molecules contained in the liquid crystal layer and the hydrophobicity parameter is controlled to be equal to or more than a specific value; and, on the basis of the correlation between the hydrophobicity parameter and a —OCF3 group at the end of the liquid crystal molecule, a method in which the content of a liquid crystal compound having an —OCF3 group at the end of the liquid crystal molecule in a liquid crystal composition is controlled to a specific value or more have been disclosed (PTL 3). However, the essence of the invention disclosed in the cited document is reducing the effect of impurities contained in a pigment on the liquid crystal layer, and there was no study on the direct relationship between the structure of a pigment used for producing a color filter and the structure of a liquid crystal material.
It has been disclosed that voltage holding ratio (VHR) may be increased by using a pigment washed with deionized water until the specific electrical conductivity of the filtrate of the deionized water used for the washing treatment reaches 20 μS/cm or less. However, there is no description about the specific electrical conductivity of the pigment, and the voltage holding ratio was about 95% (PTL 4), which was insufficient to address the faulty display of liquid crystal display elements, which are becoming more advanced.
It is known that the water-soluble content and the specific electrical conductivity of a pigment may affect the anticorrosive effect of an anticorrosive paint and ease of ejecting ink-jet ink (PTL 5 and PTL 6). However, it is not known how the combination of the water-soluble content and specific electrical conductivity of a pigment and the structure of a liquid crystal material constituting a liquid crystal layer affect faulty display of a liquid crystal display element. Thus, the issue of faulty display of liquid crystal display apparatuses, which are becoming more advanced, has not yet been addressed.